Detection of optical surface of patient interface for ophthalmic laser applications using a non-confocal configuration

ABSTRACT

An ophthalmic laser system uses a non-confocal configuration to determine a laser beam focus position relative to the patient interface (PI) surface. The system includes a light intensity detector with no confocal lens or pinhole between the detector and the objective lens. When the objective focuses the light to a target focus point inside the PI lens at a particular offset from its distal surface, the light signal at the detector peaks. The offset value is determined by fixed system parameters, and can also be empirically determined by directly measuring the PI lens surface by observing the effect of plasma formation at the glass surface. During ophthalmic procedures, the laser focus is first scanned insider the PI lens, and the target focus point location is determined from the peak of the detector signal. The known offset value is then added to obtain the location of the PI lens surface.

BACKGROUND OF THE INVENTION Field of the Invention

This invention relates to an ophthalmic laser system and method forlaser focus point calibration, and in particular, it relates to anophthalmic laser system employing a non-confocal optical system andrelated method for calibrating the depth position of the laser focuspoint.

Description of Related Art

An ophthalmic laser system generally includes a laser device thatgenerates a laser beam, such as a pulsed laser beam, and a beam deliveryoptical system that delivers a focused spot of the laser beam into apatient's eye. The beam delivery system includes a scanner sub-systemfor scanning the laser focus spot in three dimensions to produce acutting pattern within a desired volume of the eye to effectuate variousdesired treatments, such as tissue incisions. More specifically, thebeam delivery system may include the following components, some of whichare optional: a polarization beam attenuator for beam energy control, abeam sampler used to sample the beam for energy monitoring, a shutter, Zscanning optics (e.g. a lens) for changing the depth position of thelaser focus spot in the eye (the depth direction or Z direction beingsubstantially parallel to the propagation direction of the laser beamand the optical axis of the eye), X-Y scanning optics (e.g. mirrors) forscanning the laser focus spot in two transverse directions perpendicularto the depth direction, beam expander, beam rotator, various turningmirrors to change the beam direction, a focusing objective lens, andadditional optical elements after the objective lens. The variouscomponents are coupled to a control system employing a computer and/orprocessors and/or hardware circuitry.

During the ophthalmic procedure, the patient's eye is physically coupledto the beam delivery system via a disposable patent interface (PI)device, which is attached at its proximal end to a housing of the beamdelivery system and at its distal end to the surface of the eye.

The objective lens is mounted on a movement structure and moveable inthe Z direction relative to the housing in order to focus the laser beamat desired depths within the eye and to vary the depth of the focusspot. An important calibration step in the operation of an ophthalmiclaser system is to establish the position of the objective lens thatfocuses the laser beam to a known depth (Z position). In someconventional methods, calibration of the objective lens is done byplacing a reflective surface in front of the objective lens at a known Zposition relative to the beam delivery system housing, and using aconfocal detector to measure the amount of reflected light that hastraveled back through the objective lens, to determine the Z position ofthe objective lens that focuses the beam on the reflecting surface.

FIG. 1 schematically illustrates a portion of an ophthalmic laser system10 that uses a confocal detection configuration for Z positioncalibration.

More generally, confocal microscopy is a widely used tool in biologicalimaging, because it significantly improves the contrast of imagescompared to wide field microscopy, and it allows depth segmenting. Aconfocal microscope is based on a double filtering operation: a certainvolume inside the sample is selectively illuminated by a focused beam,and light originating from this focal volume is selectively observedusing a pinhole in the detection pathway. The pinhole is located in aplane conjugated with the focal plane, and suppresses light originatingfrom any location other than the focal volume. With this method, a pointof a sample can be probed with higher contrast with respect to itssurroundings. Images are built by scanning the probed focal volumeinside the sample. In typical biological media, confocal microscopyallows one to obtain clear, background free images only up to a certainpoint.

In the system shown in FIG. 1, a laser source 11, the details of whichare not shown, includes the laser device and associated opticalcomponents that are configured to produce a laser beam. A part of thelaser beam passes through a beam splitter 12, and after being reflectedby one more mirror 13, is focused by the objective lens 14. The laserlight that exits the objective lens is reflected by a referencereflective surface 18A disposed below the objective lens 14, and thereflected light travels backwards into the objective lens. The referencesurface 18A may be, for example, a surface of an optical element 18 ofthe PI (referred to as the PI lens), where the PI is physically attachedto the beam delivery system housing and remains stationary relative tothe housing. After the back-reflected light is focused by the objectivelens and reflected by the mirror 13, a part of the reflected light isreflected by the beam splitter 12 into a confocal detection assembly.The confocal detection assembly includes a lens 15 (referred to as theconfocal lens), a pinhole 16, and a two-dimensional light intensitydetector 17. The confocal lens 15 is configured to focuses a parallellight beam to the pinhole 16, and the light that passes through thepinhole is detected by the detector 17.

Referring to FIGS. 1 and 2, to calibrate the Z position of the objectivelens 14, the laser source 11 is operated to generate a parallel laserbeam. The objective lens 14 is continuously moved in the Z direction,and the back-reflected light that has traveled through the objectivelens is continuously detected by the confocal detection assembly as theobjective lens is moved. In FIG. 2, the objective lens 14 may beoptically represented by a thin lens having a focal distance f, eventhough the objective lens is typically formed of a set of lenses. Asshown in FIGS. 1 and 2, the incident laser beam is a parallel beam andis focused by the objective lens 14 to a focus point F. As the objectivelens 14 is moved in the Z direction, when the focus point F is locatedon the reflective surface 18A (i.e., the distance between the objectivelens and the reflective surface equals the focal distance of theobjective lens, see FIG. 2), the light reflected by the surface 18A isfocused by the objective lens 14 into a parallel beam. Then, theconfocal lens 15 focuses the parallel beam onto the pinhole 16, and thelight passing through the pinhole is detected by the detector 17 toproduce a signal. The confocal lens 15 and the pinhole are designed suchthat a parallel beam will be focused to a focus spot comparable to orsmaller in size than the pin hole (which is for example microns insize), so a majority of the light passes through the pinhole to bedetected by the detector 17. On the other hand, when the focus point Fis located away from the reflective surface 18A (not shown in FIG. 2),the light reflected from the surface 18A, after passing through theobjective lens 14, will not be a parallel beam. Thus, the reflected beamthat enters the confocal lens 15 is not focused onto the pinhole 16, andas a result, the amount of light that passes through the pinhole 16 toreach the detector 17 is significantly reduced. Therefore, the signaldetected at the detector 17 (referred to as the auto-Z signal) peakswhen the focus spot F of the objective lens is on the reflective surface18A. By continuously moving the objective lens 14 in the Z direction andcontinuously detecting the auto-Z signal, the position of the objectivelens that focuses the parallel incident laser beam on the reflectivesurface 18A is obtained. This process is sometimes referred to asdetecting the optical surface of the PI. Once this objective lensposition is known, the objective lens position that will place the laserfocus point at any desired depth relative the reference surface 18A canbe determined.

SUMMARY

The present invention is directed to an ophthalmic laser system andrelated method that substantially obviates one or more of the problemsdue to limitations and disadvantages of the related art.

An object of the present invention is to provide an ophthalmic lasersystem that uses a simpler structure for Z direction calibration.

Additional features and advantages of the invention will be set forth inthe descriptions that follow and in part will be apparent from thedescription, or may be learned by practice of the invention. Theobjectives and other advantages of the invention will be realized andattained by the structure particularly pointed out in the writtendescription and claims thereof as well as the appended drawings.

To achieve the above objects, the present invention provides anophthalmic laser system which includes: a laser source configured togenerate a parallel light beam; an objective lens configured to focusthe parallel light beam to a focus point; an objective lens movementstructure configured to move the objective lens in a Z direction whichis parallel to an optical path of the ophthalmic laser system; a lightintensity detector; and a beam splitter disposed to guide a portion ofthe light beam from the laser source to the objective lens and to guidea portion of a back-reflected light beam from the objective lens to thelight intensity detector, wherein the optical path is free of any lensor pinhole between the objective lens and the light intensity detector.

The ophthalmic laser system further includes a controller electricallycoupled to the light detector and the objective lens movement structure,the controller being configured to control a movement of the objectivelens and to analyze a corresponding signal detected by the lightintensity detector to determine a position of the objective lens in theZ direction.

In some embodiments, the controller is configured to: set a power of thelight beam to a first power below a plasma threshold for the patientinterface optical element, and while the power is set to the firstpower: control the objective lens movement structure to move theobjective lens in the Z direction; control the light intensity detectorto measure a light that has been reflected back by the optical surfaceof the patient interface optical element and has passed through theobjective lens; analyze a light intensity signal produced by thedetector while the objective lens is moved; and determine a firstposition of the objective lens that corresponds to a peak position inthe light intensity signal; set the power of the light beam to a secondpower above the plasma threshold for the patient interface opticalelement, and while the power is set to the second power: control theobjective lens movement structure to move the objective lens in the Zdirection to move the light focus point from a position outside of thepatient interface optical element through the optical surface to aposition inside the patient interface optical element; control the lightintensity detector to measure a light that has been reflected back bythe optical surface of the patient interface optical element and haspassed through the objective lens; and determine a second position ofthe objective lens that corresponds to a sudden decrease in the lightintensity signal; and calculate a difference between the first andsecond positions as an offset value and store the offset value.

In some embodiments, the controller is configured to: control theobjective lens movement structure to move the objective lens in the Zdirection; control the light intensity detector to measure a light thathas been reflected back by the optical surface of the patient interfaceoptical element and has passed through the objective lens; analyze alight intensity signal produced by the detector while the objective lensis moved; determine a first position of the objective lens thatcorresponds to a peak position in the light intensity signal; calculatea second position of the objective lens based on the first position, anoffset value pre-stored in the controller, and a depth value, whereinthe offset value represents a distance between the optical surface ofthe patient interface optical element and a light focus position thatcorresponds to the peak position in the light intensity signal; move theobjective lens to the second position; and operate the laser source toscan the focus point of the light beam according to a scan pattern.

In another aspect, the present invention provides a method implementedin an ophthalmic laser system, the ophthalmic laser system comprising alaser source for generating a light beam, a moveable objective lens forfocusing the light beam, a light intensity detector for detecting alight signal from the objective lens, and a controller, the methodincluding: mounting a patient interface device on the ophthalmic lasersystem, the patient interface device having an optical element with adistal optical surface; using the objective lens to focus the light beamgenerated by the laser source to a focus point located inside thepatient interface optical element; moving the objective lens to move thefocus point; using the light intensity detector, detecting aback-reflected light that has been reflected by the distal opticalsurface of the patient interface optical element and has passed throughthe objective lens, to generate a light intensity signal, wherein theback-reflected light travels from the objective lens to the lightintensity detector without passing through any other lens or anypinhole; analyzing the light intensity signal to determine a firstposition of the objective lens that corresponds to a peak position inthe light intensity signal; determining a second position of theobjective lens that focuses the light beam to the distal optical surfaceof the patient interface optical element; calculating a differencebetween the first and second positions of the objective lens as anoffset value; and storing the offset value.

The step of determining the second position of the objective lensincludes: setting a power of the light beam to be above the plasmathreshold for the patient interface optical element; moving theobjective lens to move the light focus point from a location outside ofthe patient interface optical element through the distal optical surfaceto a location inside the patient interface optical element; using thelight intensity detector, detecting a light generated at the focus pointthat has passed through the objective lens; and analyzing the lightintensity signal to determine the second position of the objective lenswhich corresponds to a sudden decrease of the light intensity signal.

In anther aspect, the present invention provides a method implemented inan ophthalmic laser system, the ophthalmic laser system comprising alaser source for generating a light beam, a moveable objective lens forfocusing the light beam, a light intensity detector for detecting alight signal from the objective lens, and a controller, the methodincluding: mounting a patient interface device on the ophthalmic lasersystem, the patient interface device having an optical element with adistal optical surface; using the objective lens to focus the light beamgenerated by the laser source to a focus point located inside thepatient interface optical element; moving the objective lens to move thefocus point; using the light intensity detector, detecting aback-reflected light that has been reflected by the optical surface andhas passed through the objective lens, to generate a light intensitysignal, wherein the back-reflected light travels from the objective lensto the light intensity detector without passing through any other lensor any pinhole; analyzing the light intensity signal to determine afirst position of the objective lens that corresponds to a peak positionin the light intensity signal; calculating a second position of theobjective lens based on the first position, a pre-stored offset value,and a depth value, wherein the offset value represents a distancebetween the distal optical surface of the patient interface opticalelement and a light focus position that corresponds to the peak positionin the light intensity signal; moving the objective lens to the secondposition; and operating the laser source to scan the focus point of thelight beam according to a scan pattern.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory and areintended to provide further explanation of the invention as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically illustrates a portion of a conventional ophthalmiclaser system that uses a confocal detection configuration forcalibrating the Z position of the objective lens.

FIG. 2 schematically illustrates the principle of calibrating the Zposition of the objective lens in the laser system of FIG. 1.

FIG. 3 schematically illustrates a portion of an ophthalmic laser systemthat uses a non-confocal detection configuration for calibrating the Zposition of the objective lens according to an embodiment of the presentinvention.

FIGS. 4A and 4B schematically illustrate the principle of calibratingthe Z position of the objective lens in the laser system of FIG. 3.

FIG. 5 schematically illustrates a method for calibrating the Z positionof the objective lens in the laser system of FIG. 3.

FIG. 6 schematically illustrates a method for calibrating the Z positionof the objective lens and performing ophthalmic procedure using thelaser system of FIG. 3.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

As described above, the conventional optical system for calibrating theZ position of the objective lens uses a confocal detection assemblyincluding a lens and a pinhole in front of the detector. Such a confocalsystem requires precise alignment of the confocal lens to the pinhole,as the confocal lens must focus the waist of the light precisely at thepinhole. This makes the system relatively complex in terms of bothmanufacturing of the precision components and the alignment of thecomponents.

Embodiments of the present invention provide a simpler optical systemfor calibrating the Z position of the objective lens, which uses a lensimaging principle that does not use a confocal configuration.

FIG. 3 schematically illustrates a portion of an ophthalmic laser system30 that uses a non-confocal detection configuration for Z positioncalibration according to an embodiment of the present invention. In thesystem shown in FIG. 3, the laser source 31, the details of which arenot shown, includes a laser device and associated optical componentsthat are configured to produces a laser beam. A part of the laser beampasses through a beam splitter 32, and after being reflected by one moremirrors 33, is focused by the objective lens 34. In a preferredembodiment, the objective lens 34 has a relatively high numericalaperture (NA), for example, approximately 0.4 or higher. The focus spotsize produced by the objective lens is preferably as small as 2 μm, or 1μm, or even smaller.

The objective lens 34 is mounted on a movement structure and moveable inthe Z direction relative to the housing in order to focus the laser beamat desired depths and to vary the depth of the focus spot. The movementstructure may include any suitable mechanical structure, such as atranslation stage driven by a motor, etc.

A part of the laser light that exits the objective lens 34 is reflectedby a reference reflective surface 38A disposed below the objective lens34, and the reflected light travels backwards into the objective lens34. After the back-reflected light is focused by the objective lens 34and reflected by the mirror 33, a part of the reflected light isreflected by the beam splitter 32 onto a small two-dimensional lightintensity detector 37 (e.g. a photodetector). No confocal lens orpinhole is used in front of the detector 37.

In preferred embodiments, the reference reflective surface 38A is thedistal surface of an optical element 38 of the PI (referred to as the PIlens), i.e. one of the two surface of the PI lens that is locatedfarther away from the objective lens. The PI lens may be a piece of flatglass, or it may have one or two curved surfaces. The PI is physicallyattached to the beam delivery system housing and remains stationaryrelative to the housing. FIG. 3 schematically illustrates a part of thehousing 39, and the PI lens 38, but the rest of the housing and the PIare not shown. While the PI lens is transparent, in practice, itssurfaces reflect a small portion (e.g. a few percent) of the beam backto the optical system. This reflected beam is used to calibrate the Zposition of the objective lens.

The principle of Z position calibration in the laser system of FIG. 3 isdescribed below with reference to the schematic illustration in FIGS. 4Aand 4B. In FIGS. 4A and 4B, the objective lens 34 is opticallyrepresented by a thin lens having a focal distance f, although theobjective lens is typically formed of a set of lenses. It should benoted that FIGS. 4A and 4B are intended to explain the relevant opticalprinciples; the various distances depicted in the figures are not toscale.

FIG. 4A illustrates a situation where the objective lens 34 focuses theparallel incident beam to a focus point F located between the objectivelens 34 and the distal surface 38A of the PI lens 38, and inside the PIlens. In other words, the distal surface 38A is located beyond the focalplane of the objective lens 34. The forward propagating light divergesafter the focus point F and is then reflected by the reflective surface38A. To the objective lens 34, the reflected light appears to originatefrom a point B behind the reflective surface 38A, the point B being themirror image of the focus point F with respect to the reflective surface38A. The distance from the equivalent origin B to the objective lens 34is u=ƒ+2δ (Equation (1)), where δ=FA is the offset between the focuspoint F and the distal surface 38A of the PI lens. The reflected lightfrom the equivalent origin B is focused by the objective lens 34 to animage point D at a finite distance v from the objective lens. Thedetector 37 is located at the image point D, and no other lens isdisposed between the objective lens 34 and the detector 37. In the lasersystem 30, the distance DA between the location D of the detector 37 andthe reflective surface 38A is a constant because both the detector andthe reflective surface 38A are fixed with respect to the system housing,i.e., DA=v+ƒ+δ, or DA≈v+ƒ (Equation (2)) when δ is much smaller thanboth v and ƒ (discussed later) and can be ignored. In other words, v canbe treated as a fixed value determined by the hardware configuration.

Using Equations (1) and (2) and the following lens formula for a thinlens (Equation (3)),

${{\frac{1}{u} + \frac{1}{v}} = \frac{1}{f}},$where u is the object distance and v is the image distance, and takinginto consideration that δ is much smaller than both v and ƒ and that ƒis much smaller than v (discussed later), one obtains (Equation (4)):

$\delta \approx \frac{f^{2}}{2\; v}$The above equations are for focusing in the air. Since the focus point Fis located inside the PI lens 38, the refractive index n of the PI lensmaterial is taken into consideration, and one obtains (Equation (5)):

$\delta \approx {n \cdot \frac{f^{2}}{2\; v}}$

The point inside the PI lens 38 at the distance δ from the distalsurface 38A, where δ satisfies Equation (5), is referred to as thetarget focus position for convenience.

It should be understood that in the above equations, the variousdistances are the distances along the optical path; the optical path maybe folded by mirrors or beam splitters.

In some embodiments of the laser system 30, the focal distance ƒ, i.e.the equivalent focal length of the objective lens 34, is a few mm, e.g.approximately 4 mm. Meanwhile, the distance DA from the detector 37 tothe distal surface 38A of the PI lens, i.e., DA=v+ƒ (ignoring δ), may beseveral hundreds of mm, because the choice of the detector location isnot constrained and the image distance v may be lengthened if desired byfolding the optical path with mirrors. In one particular embodiment,where the image distance v is approximately 500 mm, the equivalent focallength ƒ is 3.92 mm, and the refractive index of the PI lens (glass) isn=1.45, Equation (5) gives δ≈22 μm. These values confirm that δ is muchsmaller than both v and ƒ (by at least a factor on the order of 100) andthat ƒ is much smaller than v (by a factor on the order of 100).

When the objective lens 34 focuses the laser beam at positions otherthan the target focus position, the back-reflected light will not befocused on the detector 37 at the point D, but will be focused beforeit, after it, or not be focused at all. FIG. 4B schematicallyillustrates an example where the focus point F′ is located beyond thereflective surface 38A, i.e. outside of the PI lens 38. The light fromthe objective lens 34 converges as it strikes the reflected surface 38Aand is reflected by it; therefore, to the objective lens 34, theback-reflected light appears to originate from a point B′ in front ofthe reflective surface 38A, the point B′ being the mirror image of thefocus point F′ with respect to the reflective surface 38A. Since thedistance from the point B′ to the objective lens 34 is shorter than thefocal distance, the back-reflected light remains divergent after itpasses through the lens 34. As illustrated in FIG. 4B, the distance fromthe focus point F′ to the reflective surface 38A is denoted A; theback-reflected light has an object distance u=ƒ−2Δ, and forms a virtualimage at a point B′ behind the lens 34.

To summarize, the back-reflected light from the distal surface 38A ofthe PI lens 38 will only form a well focused real image on the detector37 when the objective lens 34 focuses the parallel beam to the targetfocus position at S. The detector 37 has a relatively small effectivedetection area, such as about 1 mm² or smaller. Having a small effectivedetection area refers to the detector either having a physically smalldetection area or being controlled to detect light falling within asmall area. As a result, when the back-reflected light is not focused onthe detector, substantial portions of the reflected light will not fallon the effective detection area and the detected light intensity willdecrease significantly. Therefore, the light intensity signal detectedat the detector 37 (the auto-Z signal) peaks when the objective lensfocuses the parallel beam to the target focus position. Thus, bycontinuously moving the objective lens 34 in the Z direction andcontinuously detecting the auto-Z signal, the position of the objectivelens that corresponds to the target focus position is determined. Thesystem can achieve a sub-micron depth resolution of the Z positiondetection. As noted earlier, the objective lens 34 is typically formedof a set of lenses even though it is optically represented by a thinlens in the above analysis. Accordingly, moving the objective lens inthe Z direction may include moving one or more of the lenses in the setof lenses so that the optical effect is that the thin lens representingthe objective lens is moved in the Z direction.

Once the objective lens position corresponding to the target focus pointis known, and given the knowledge of the value δ, the objective lensposition that places the laser focus point at the distal surface 38A ofthe PI lens can be determined, since the focus point is moved by thesame amount as the movement of the objective lens. Further, theobjective lens position that will place the laser focus point at anydesired depth relative to the distal surface 38A of the PI lens can alsobe determined.

From FIG. 3, it can be seen that the principle of Z position calibrationapplies equally when the reflective surface 38A is the proximate surfaceof the PI lens, in which case the target focus point F is located in theair at a distance δ from the proximate surface. But because the depthsof the anatomical structures of the eye are more directly related to thedistal surface of the PI lens, it is more desirable to measure thetarget focus point relative to the distal surface. For a given lasersystem, the value δ that defines the target focus position is fixed, asit is related to the focal length f of the objective lens and the fixeddistance DA from the detector 37 to the surface 38A by Equation (5).While in principle the value δ for a given laser system can becalculated from Equation (5), in practice, the fixed distance DA may becumbersome to measure. Thus, rather than calculating the value δ fromEquation (5), the value δ may be empirically determined using thefollowing process. In this method, the location of the distal surface ofthe PI lens is determined by using a high power beam to cause plasmaformation at the surface of the PI lens.

Referring to FIG. 5, first, the objective lens position corresponding tothe target focus point is determined by moving the objective lens in theZ direction and recording the auto-Z signal measured by the detector 37,as described earlier (step S51). The objective lens positioncorresponding to the peak of the auto-Z signal is determined and denotedZ₁. In this step, the laser power is set to be below the plasmathreshold for the glass of the PI lens. Preferably, the objective lensposition is moved in the deep-to-shallow direction (i.e. in the upwarddirection in the orientation of FIG. 3). For example, the laser focuspoint may be initially placed outside the PI lens 38 beyond the distalsurface 38A, and the objective lens is moved in an appropriate directionto move the focus point upwards until the focus point moves into the PIlens and past the target focus position, while the peak of the detectorsignal is observed to determine Z₁.

Then, the laser power is set to be above and near the plasma thresholdfor the glass of the PI lens, and the objective lens is initially set ata position that places the laser focus point outside of the PI lens 38(beyond the surface 38A, i.e. in the air) and then moved in thedeep-to-shallow direction, while the detector signal is recorded (stepS52). When the laser focus point is moved from air into the PI lens, thedetector signal will suddenly fall when the laser focus touches theglass surface due to plasma formation in the glass. For example, thesignal may be slow-varying and then suddenly fall by 15% or more (forexample, as much as 40% in some instances) within a Z range of 1 μm.This sudden fall is believed to be caused by the damage (burn) at theglass surface which scatters the light. The objective lens positioncorresponding to the sudden fall of the detector signal is determinedand denoted Z₂ (step S52). This position corresponds to the location ofthe distal surface of the PI lens. The value δ is calculated as thedifferences between the two positions of the objective lens: δ=Z₂−Z₁(step S53).

The above method shown in FIG. 5 is performed after the laser apparatusis manufactured, and may be performed from time to time during systemmaintenance, but is not performed for each ophthalmic procedure. Duringan actual ophthalmic procedure, it is only necessary to determine theobjective lens position that corresponds to the target focus position.More specifically, as shown in FIG. 6, after the PI is coupled to thepatient's eye and to the housing of the laser system 30 (step S61), thelaser power is set to a low level that does not cause any tissuemodification in the eye (step S62), and the objective lens 34 is movedwhile the reflected light signal is detected by the detector 37 (stepS63). The detected signal is analyzed to identify the objective lensposition (denoted Z₁) that correspond to the peak of the detected signal(step S64). Based on the position Z₁ and the known value δ, which hasbeen determined beforehand and stored in the system, the objective lensposition Z₀ that will place the laser focus point at the distal surfaceof the PI lens can be calculated by Z₀=Z₁+δ, and the objective lensposition Z that will place the laser focus point at a desired depth dbelow the distal surface of the PI lens can be calculated byZ=Z₁+δ+d=Z₀+d. This relationship is used to move the objective lens toany desired depth in the eye (step S65). A scanner assembly in the lasersource 31 can then be operated to scan the focus point in threedimensions within the eye according to desired scan patterns (step S66).

The above methods may be performed automatically using a controller 40which is electrically coupled to the laser source 31, the detector 37,and the movement structure of the objective lens 34. The controller mayinclude suitable electrical circuitry, and/or microprocessors, and/or acomputer, along with associated memory storing computer-readable programinstructions.

An advantage of the system shown in FIG. 3 is that it eliminates thepinhole and confocal lens of the confocal detection system. Thisincreases the robustness of the system as it uses fewer components, andrelaxes the associated alignment requirement. Even though it does notuse a confocal system, the system still has the flexibility that thedetector can be located at any convenient location along the opticalpath. The system can achieve approximately the same precision in Zposition determination as using a confocal system.

The glass burning effect, i.e. the phenomenon of light behavior changedue to plasma formation at the glass surface, described above inconnection with step S52 of the Z position calibration method, may alsobe used for other purposes in an ophthalmic laser system.

For example, the glass burning phenomenon may be used by itself todetermine the position of the PI lens surface. In other words, step S52described above may be performed, without performing step S51, todetermine the objective lens position which corresponds to the focuspoint being placed at the PI lens surface. This position may then beused as a reference position in actual ophthalmic procedure to focus thelaser beam to any desired depth below the PL lens surface. This methodmay be employed in an ophthalmic laser system that uses a non-confocalconfiguration such as that shown in FIG. 3, or a confocal configurationsuch as that shown in FIG. 1.

Moreover, this method may even be used with an external photodetector,i.e., a detector that is not located inside the physical housing of theophthalmic laser system, by pointing the photodetector at the surface ofthe PI lens. It has been observed that when the laser focus point ismoved from the air and reaches the glass surface, the light emitted fromthe glass surface in other directions (not just toward the objectivelens) changes suddenly. This sudden change may be detected using theexternal photodetector to determine the Z position of the objective lensthat focuses the beam at the PI lens surface.

In another example, the glass burning phenomenon may be used to checkthe health of the laser system. In a well maintained laser system, thethreshold values of various system parameters, such as the pulse powersetting, that would produce glass burning of the PI lens can bedetermined. Then, at a later time, such as after the system is shippedto a different location, the glass burning procedure is repeated at thethreshold values of the system parameters. If the glass burning occursas expected, then it indicates that the laser system is in optimumcondition. Conversely, if the glass burning does not occur, then itindicates that the laser system is not in optimum condition. Such aprocedure may be performed using a confocal configuration ornon-confocal configuration, or using an external photodetector asdescribed above.

The optical system with a non-confocal configuration as shown in FIG. 3may also be used to achieve a microscope imaging system that does notemploy a confocal lens and a pinhole. Similar to the confocal microscopytechnique, the objective lens focuses the illumination light to a smallvolume inside the sample, and light originating from this focal volumeis focused by the objective lens onto the small detector, without usinga confocal lens or pinhole. Images are built by scanning the probedfocal volume inside the sample. This technique can achieve similarresults as confocal microscopy. The technique is applicable for high NAobjective lens microscope since such objective lens decrease the depthof field. Thus, for a very small focal displacement, a very large beamdiameter is expected at the detector plane. As a result, only reflectedlight from a very small focal field is detected by the detector,achieving a similar effect as confocal microscopy.

It will be apparent to those skilled in the art that variousmodification and variations can be made in the ophthalmic laser systemand related Z position calibration method of the present inventionwithout departing from the spirit or scope of the invention. Thus, it isintended that the present invention cover modifications and variationsthat come within the scope of the appended claims and their equivalents.

What is claimed is:
 1. An ophthalmic laser system comprising: a lasersource configured to generate a parallel light beam; an objective lensconfigured to focus the parallel light beam to a focus point; anobjective lens movement structure configured to move the objective lensin a Z direction which is parallel to an optical path of the ophthalmiclaser system; a light intensity detector; a beam splitter disposed toguide a portion of the light beam from the laser source to the objectivelens and to guide a portion of a back-reflected light beam, which hasbeen reflected back by a reference reflective surface disposed in frontof the objective lens and has passed through the objective lens, to thelight intensity detector, wherein the reference reflective surface is asurface attached to the laser system other than any anatomical structureof an eye of a patient, wherein the optical path is free of any lens orpinhole between the objective lens and the light intensity detector; acontroller electrically coupled to the light detector and the objectivelens movement structure, the controller being configured to control amovement of the objective lens and to analyze a corresponding lightintensity signal detected by the light intensity detector, whichrepresents an intensity of the light which has been reflected back bythe reference reflective surface and is incident on the light intensitydetector, including to determine that a position of the objective lensin the Z direction is at a predefined position with respect to thereference reflective surface by determining a peak of the lightintensity signal detected by the light intensity detector.
 2. Theophthalmic laser system of claim 1, wherein the light intensity detectorhas an effective detection area of 1 mm² or less.
 3. The ophthalmiclaser system of claim 1, wherein the objective lens is configured tofocus the parallel light beam to the focus point having a size of 2 μmor less.
 4. The ophthalmic laser system of claim 1, wherein theobjective lens has a numerical aperture of 0.4 or higher.
 5. Theophthalmic laser system of claim 1, further comprising: a housing,wherein the light intensity detector is mounted in the housing at afixed location, and wherein the objective lens is moved relative to thehousing; and a patient interface optical element mounted on the housingand having an optical surface disposed to receive the light beam fromthe objective lens and to reflect a part of the received light beam backto the objective lens, wherein the optical surface of the patientinterface optical element forms the reference reflective surface.
 6. Anophthalmic laser system comprising: a laser source configured togenerate a parallel light beam; an objective lens configured to focusthe parallel light beam to a focus point; an objective lens movementstructure configured to move the objective lens in a Z direction whichis parallel to an optical path of the ophthalmic laser system; a lightintensity detector; a housing, wherein the light intensity detector ismounted in the housing at a fixed location, and wherein the objectivelens is moved relative to the housing; a patient interface opticalelement mounted on the housing and having an optical surface disposed toreceive the light beam from the objective lens and to reflect a part ofthe received light beam back to the objective lens, wherein the opticalsurface of the patient interface optical element forms a referencereflective surface; a beam splitter disposed to guide a portion of thelight beam from the laser source to the objective lens and to guide aportion of a back-reflected light beam, which has been reflected back bythe reference reflective surface disposed in front of the objective lensand has passed through the objective lens, to the light intensitydetector, wherein the optical path is free of any lens or pinholebetween the objective lens and the light intensity detector; and acontroller electrically coupled to the light detector, the objectivelens movement structure and the laser source, the controller beingconfigured to: set a power of the light beam to a first power below aplasma threshold for the patient interface optical element, and whilethe power is set to the first power: control the objective lens movementstructure to move the objective lens in the Z direction; control thelight intensity detector to measure a light that has been reflected backby the optical surface of the patient interface optical element and haspassed through the objective lens; analyze alight intensity signalproduced by the detector while the objective lens is moved; anddetermine a first position of the objective lens that corresponds to apeak position in the light intensity signal; set the power of the lightbeam to a second power above the plasma threshold for the patientinterface optical element, and while the power is set to the secondpower: control the objective lens movement structure to move theobjective lens in the Z direction to move the light focus point from aposition outside of the patient interface optical element through theoptical surface to a position inside the patient interface opticalelement; control the light intensity detector to measure a light thathas been reflected back by the optical surface of the patient interfaceoptical element and has passed through the objective lens; and determinea second position of the objective lens that corresponds to a suddendecrease in the light intensity signal; and calculate a differencebetween the first and second positions as an offset value and store theoffset value.
 7. The ophthalmic laser system of claim 6, wherein theoptical surface is a distal optical surface of the patient interfaceoptical element.
 8. An ophthalmic laser system comprising: a lasersource configured to generate a parallel light beam; an objective lensconfigured to focus the parallel light beam to a focus point; anobjective lens movement structure configured to move the objective lensin a Z direction which is parallel to an optical path of the ophthalmiclaser system; a light intensity detector; a housing, wherein the lightintensity detector is mounted in the housing at a fixed location, andwherein the objective lens is moved relative to the housing; a patientinterface optical element mounted on the housing and having an opticalsurface disposed to receive the light beam from the objective lens andto reflect a part of the received light beam back to the objective lens,wherein the optical surface of the patient interface optical elementforms the reference reflective surface; a beam splitter disposed toguide a portion of the light beam from the laser source to the objectivelens and to guide a portion of a back-reflected light beam, which hasbeen reflected back by a reference reflective surface disposed in frontof the objective lens and has passed through the objective lens, to thelight intensity detector, wherein the optical path is free of any lensor pinhole between the objective lens and the light intensity detector;and a controller electrically coupled to the light detector, theobjective lens movement structure and the laser source, the controllerbeing configured to: control the objective lens movement structure tomove the objective lens in the Z direction; control the light intensitydetector to measure a light that has been reflected back by the opticalsurface of the patient interface optical element and has passed throughthe objective lens; analyze alight intensity signal produced by thedetector while the objective lens is moved; determine a first positionof the objective lens that corresponds to a peak position in the lightintensity signal; calculate a second position of the objective lensbased on the first position, an offset value pre-stored in thecontroller, and a depth value, wherein the offset value represents adistance between the optical surface of the patient interface opticalelement and a light focus position that corresponds to the peak positionin the light intensity signal; move the objective lens to the secondposition; and operate the laser source to scan the focus point of thelight beam according to a scan pattern.
 9. The ophthalmic laser systemof claim 8, wherein the offset value is a fixed value determined by afocal length of the objective lens and a fixed length of the opticalpath from the light intensity detector to the optical surface of thepatient interface optical element.
 10. The ophthalmic laser system ofclaim 8, wherein the optical surface is a distal optical surface of thepatient interface optical element.
 11. A method implemented in anophthalmic laser system, the ophthalmic laser system comprising a lasersource for generating a light beam, a moveable objective lens forfocusing the light beam, a light intensity detector for detecting alight signal from the objective lens, and a controller, the methodcomprising: mounting a patient interface device on the ophthalmic lasersystem, the patient interface device having an optical element with adistal optical surface; using the objective lens to focus the light beamgenerated by the laser source to a focus point located inside thepatient interface optical element; moving the objective lens to move thefocus point; using the light intensity detector, detecting aback-reflected light that has been reflected by the distal opticalsurface of the patient interface optical element and has passed throughthe objective lens, to generate alight intensity signal, wherein theback-reflected light travels from the objective lens to the lightintensity detector without passing through any other lens or anypinhole; analyzing the light intensity signal to determine a firstposition of the objective lens that corresponds to a peak position inthe light intensity signal; determining a second position of theobjective lens that focuses the light beam to the distal optical surfaceof the patient interface optical element; calculating a differencebetween the first and second positions of the objective lens as anoffset value; and storing the offset value.
 12. The method of claim 11,wherein the step of determining a second position of the objective lensincludes: setting a power of the light beam to be above a plasmathreshold for the patient interface optical element; moving theobjective lens to move the light focus point from a location outside ofthe patient interface optical element through the distal optical surfaceto a location inside the patient interface optical element; using thelight intensity detector, detecting a light generated at the focus pointthat has passed through the objective lens; and analyzing the lightintensity signal to determine the second position of the objective lenswhich corresponds to a sudden decrease of the light intensity signal.13. The method of claim 11, wherein the light intensity detector has aneffective detection area of 1 mm² or less.
 14. The method of claim 11,wherein the focus point has a size of 2 μm or less.
 15. The method ofclaim 11, wherein the objective lens has a numerical aperture of 0.4 orhigher.
 16. A method implemented in an ophthalmic laser system, theophthalmic laser system comprising a laser source for generating a lightbeam, a moveable objective lens for focusing the light beam, a lightintensity detector for detecting a light signal from the objective lens,and a controller, the method comprising: mounting a patient interfacedevice on the ophthalmic laser system, the patient interface devicehaving an optical element with a distal optical surface; using theobjective lens to focus the light beam generated by the laser source toa focus point located inside the patient interface optical element;moving the objective lens to move the focus point; using the lightintensity detector, detecting a back-reflected light that has beenreflected by the distal optical surface of the patient interface opticalelement and has passed through the objective lens, to generate a lightintensity signal, wherein the back-reflected light travels from theobjective lens to the light intensity detector without passing throughany other lens or any pinhole; analyzing the light intensity signal todetermine a first position of the objective lens that corresponds to apeak position in the light intensity signal; calculating a secondposition of the objective lens based on the first position, a pre-storedoffset value, and a depth value, wherein the offset value represents adistance between the distal optical surface of the patient interfaceoptical element and a light focus position that corresponds to the peakposition in the light intensity signal; moving the objective lens to thesecond position; and operating the laser source to scan the focus pointof the light beam according to a scan pattern.
 17. The method of claim16, wherein the light intensity detector has an effective detection areaof 1 mm² or less.
 18. The method of claim 16, wherein the focus pointhas a size of 2 μm or less.
 19. The method of claim 16, wherein theobjective lens has a numerical aperture of 0.4 or higher.